The sole, usually very spacious, is divided into two parts, of which the one farthest off from the furnace is a little higher than the other. Above the vault there is a space or chamber in which the ore is deposited, and which communicates with the laboratory by a vertical passage; which serves to allow the ore to be pushed down, when it is dried and a little heated. The flame and the smoke which escape from the sole or laboratory pass into condensing chambers, before entering into the chimney of draught, so as to deposit in them the oxide of arsenic and other substances. When the ore on the part of the sole farthest from the grate has suffered so much heat as to begin to be roasted, has became less fusible, and when the roasting of that in the nearer part of the sole is completed, the former is raked towards the fire-bridge, and its ustulation is finished by stirring it over frequently with a paddle, skilfully worked, through one of the doors left in the side for this purpose. The operation is considered to be finished when the vapours and the smell have almost wholly ceased; its duration depending obviously on the nature of the ores.
When this furnace is employed to roast very arsenical ores, as the tin ores of Schlackenwald in Bohemia, and at Ehrenfriedersdorf in Saxony, the arsenical pyrites of Geyer (in Saxony), &c., the chambers of condensation for the arsenious acid are much more extensive than in the furnaces commonly used for roasting galena, copper, or even silver ores.
[Figs. 686], [687], [688.] represent a reverberatory furnace employed in the smelting works of Lautenthal, in the Hartz, for roasting the schlichs of lead ores, which contain much blende or sulphuret of zinc. In [fig. 686.] we see that the two parts A B, B C, are absolutely like, the two furnaces being built in one body of brickwork. [Fig. 687.] is the plan of the furnace B C, taken at the level E F of [fig. 686.] [Fig. 688.] is a vertical section of the similar furnace A B, taken in the prolongation of the line G H in [fig. 687.]
a is the fire-place of the furnace, its grate and ash-pit. b is the conduit of vaporization, which communicates with the chambers c; c, chambers into which the vaporized substances are deposited; d, chimney for the escape of the smoke of the fire-place a, after it has gone through the space b c c; e′, is the charging door, with a hook hanging in front to rest the long iron rake upon, with which the materials are turned over; f, chamber containing a quantity of schlich destined for roasting; this chamber communicates with the vaulted corridor (gallery) D, seen in [fig. 686.]; g, orifice through which the schlich is thrown into the furnace; h, area or hearth of the reverberatory furnace, of which the roof is certainly much too high; i, channels for the escape of the watery vapours; k l, front arcade, between which and the furnace, properly speaking, are the two orifices of the conduits, which terminate at the channels m, m′. m is the channel for carrying towards the chimney d, the vapours which escape by the door e′. n is a walled-up door, which is opened from time to time, to take out of the chambers c, c, the substances that may be deposited in them.
At the smelting works of Lautenthal, in such a roasting furnace, from 6 to 9 quintals (cwts.) of schlich are treated at a time, and it is stirred frequently with an iron rake upon the altar h. The period of this operation is from 6 to 12 hours, according as the schlich may be more or less dry, more or less rich in lead, or more or less charged with blende. When the latter substance is abundant, the process requires 12 hours, with about 60 cubic feet of cleft billets for fuel.
In such furnaces are roasted the cobalt ores of Schneeberg in Saxony, the tin ores of Schlackenwald in Bohemia, of Ehrenfriedersdorf in Saxony, and elsewhere; as also the arsenical pyrites at Geyer in Saxony. But there are poison towers and extensive condensing chambers attached in the latter case. See [Arsenic].